My friend Dirk sent me this link to a New York Times article about the efficiency benefits of having two monitors attached to your computer. If you're looking for data to justify the expense of a two monitor configuration, read this article.
I've been touting the benefits of using two monitors for a long time. I use a dual-monitor setup in my office, and believe that once you've tried it, there's no going back to a single monitor.
20-24 inch wide screen flat panel displays from vendors such as Dell have fallen in price to the point that for many people it's no longer the display expense that is the issue. It's more an issue of clearing up desk space and making sure that the monitors match up with the output capabilities of the computer.